Semiconducting glaze compositions are used to provide a controlled surface resistance on the insulator so that a leakage current flow through the glaze raises the surface temperature of the insulator by a few degrees above ambient at normal operating voltages. This temperature rise of the insulator surface prevents moisture condensation and moisture deposition caused by the hygroscopic nature of some contaminants. This reduces the tendency for electrical discharge.
The semiconducting glaze compositions used for this purpose generally contain a base glaze in which various metal oxides have been incorporated. The composition is normally applied to the surface of the insulator, or to an unfired ceramic body of the insulator, as an aqueous slurry and then fired into the surface.
The surface resistivity of a semiconducting glaze coating applied to a high voltage insulator should be 10-200 M.OMEGA./ square. In order to produce a semiconducting glaze coating having such a surface resistivity, it has been proposed to add various conducting metal oxides to a conventional ceramic glaze composition. One such metal oxide proposed is ferric oxide but its use on an insulator exposed to heavily polluted atmosphere renders the insulator liable to electrolytic corrosion. The glaze also has a tendency to be thermally unstable and the appearance of this glaze is an unfavorable black.
A second metal oxide proposed for use in the semiconducting glaze compositions is titanium oxide. Such glazes, however, are damaged by discharges resulting in the loss of conductivity due to re-oxidation of the titania. Additionally, the conditions of preparation, particularly the firing conditions, must be strictly controlled and the process of glazing with this semiconducting glaze composition is complicated. As a result, this glaze composition is not ordinarily employed.
Most semiconducting glaze compositions use a combination of stannic oxide with a small amount of antimony pentoxide or antimony trioxide in a conventional porcelain glaze base. The slurry so obtained is applied to "green" procelain insulator shells by dipping, spraying or flooding and the insulator shells are fired in a pre-set cycle which matures the glaze and porcelain providing, as an end result, a glaze with a controlled surface resistivity and a porcelain body with the required electrical and mechanical strength.
Aside from the resistivity of these glazes, an important additional electrical characteristic which must be provided for is a low temperature coefficient of resistivity, so that the glaze resistivity does not significantly change with increased or decreased ambient temperatures. This temperature coefficient is commonly expressed as the half temperature (T.sup.1/2) which is defined to be the temperature interval in degrees Centigrade in which the resistivity of the glaze drops to one-half of its initial value. Thus, a high T.sup.1/2 denotes a low temperature coefficient of resistivity.
It has been found that in order to provide a low temperature coefficient of resistivity, i.e., a high T.sup.1/2, stannic oxide-antimony pentoxide (or trioxide) glazes, the loading of the semiconducting phase (stannic oxideantimony oxide) must be low, the particle size distribution of the stannic oxide must be narrow and sub-micron in range, and the firing cycle must be very carefully controlled. Control in the firing cycle includes both the time and temperature of the peak soaking period as well as the final cooling rate of the glaze. Some increase in T.sup.1/2 may be achieved by changing the composition of the base glaze if the firing cycle is optimal and also very carefully controlled. When all of the processing variables are optimized, glazes with a T.sup.1/2 of up to 200.degree. C. can be obtained. However, these glazes are extremely sensitive to firing variables.
I have now found a new semiconductive glaze composition which is particularly useful on high voltage ceramic insulators, which is much less sensitive to firing variables and has a greatly improved low temperature coefficient of resistivity. Indeed, glazes with T.sup.1/2 which are double and triple those obtained using stannic oxide can be obtained routinely. Advantageously, the same procedures as employed to prepare known glaze compositions can be used to prepare the glaze compositions of this invention.
Accordingly, it is the object of this invention to provide a novel semiconducting glaze composition suitable for use on high voltage ceramic insulators which have a high half temperature. This and other objects of the invention will become apparent to those skilled in this art from the following detailed description.